# Why is the Rainbow a Semicircle?

Discussion in 'Earth Science' started by tavilach, Dec 6, 2003.

1. ### tavilachRegistered Member

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I'm doing a project for my Hebrew class (Jewish school) about why the rainbow is a semicircle, and I can't figure out the scientific answer, let alone how to say it in Hebrew!

Can someone explain to me the exact scientific reason behind the rainbow's shape? And why, when the sun is closer to the horizon, it gets bigger? Does this half to do with the angle of refraction?

I'm very confused, any help would be lovely.

3. ### SkinWalkerArchaeology / AnthropologyModerator

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I think the answer is that it only looks like a semicircle.

I've seen full circle rainbows, though this is more rare. The circular effect probably has to do with the shape of the raindrops that the sun's light is being reflected or refracted in.

I would assume that a semicircle is what we see because the Earth gets in the way.

5. ### OzymandiasUnregistered UserRegistered Senior Member

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Think of a rainbow like a halo over the earth -- it's a full circle, but from our point of view we can only see half of it...

I heard of one person during a storm who got in a helicopter and flew above the storm, and actually got a photograph of a full-circle rainbow. Once-in-a-lifetime shot, he said.

The full-circle rainbow is easily duplicated with a hose...just put on a spray nozzle and spray -- then look closely, you should be able to see a full circle.

7. ### river-windValued Senior Member

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it is a full circle, because the perfectly spherical water droplets in the air split the white sunlight into the spectrum which you see as a rainbow. The light comes in at an angle, and then reflects out at an angle. From your perspective on the ground, that angle exsists in a full 360 degree arc - a circle. You only see a semi-circle because either the earth gets in the way of seeing the rest, or because there isn't sufficient water suspended in the atmosphere to see the whole thing.:

http://www.unidata.ucar.edu/staff/blynds/rnbw.html

8. ### BigBlueHeadGreat Tealnoggin!Registered Senior Member

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On days in the spring where it is still cold enough that there are high-altitude ice crystals, you can sometimes look up an noon and see a rainbow that goes all the way around the sun. This is extremely rare.

More generally we see things like sun-dogs, which are refractive rainbows made by clouds to either side of the sun. These are pretty common.

9. ### NightCrawlerRegistered Member

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I think the only reason u see a semicircle of the rainbow is because your your only at one part of the world and say if u where in space and a rainbow was to apear i think u would see a full circle.

10. ### DinosaurRational SkepticValued Senior Member

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Full circle rainbows are called glories. While rare in nature, I think they can be seen from a mountain top under just the right conditions.

The poster who talked about a glory due to the spray from a hose was on target. My apartment complex has a golf course. I often see a small glory while walking down the fairway when the sprinkler system is turned on.

Those posts who mention the Earth getting in the way of a full rainbow are at least partially correct. I am not sure that you can claim that the rest of the rainbow is below the horizon. The observer, the sun, and the rain must be in certain positions with respect to each other. I am not sure that the relationship would be just right to make a glory as big as the rainbow you typically see.

11. ### hotsexyangelprincessWMDRegistered Senior Member

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maybe the light hits the ground, bounces, then stops...
hows that for an elementary school answer? :m:

12. ### curioucityUnbelievable and oddRegistered Senior Member

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I agree with the idea that the earth prevents us from seeing a full-circle rainbow (besides, if it appears in full-circle, we should have called it raincle

)
I actually think of saying that we tend to be watching the center of the rainbor (circle) everytime we look at it, but the theory seems just not right.....

13. ### James RJust this guy, you know?Staff Member

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A rainbow is produced when light from the sun is reflected by raindrops. So, whenever you see a rainbow, the sun will be behind you. The particular angle you see it at is determined by the refractive index of the water droplets, which is always approximately the same (around 43 degrees, from memory).

Since different raindrops will appear at the appropriate angle for different people, that means that the rainbow you see is different from the rainbow a person standing next to you sees. It's a nice thought that no two people see the same rainbow - if you see a rainbow it's yours and yours alone!

It is possible to see a complete circular rainbow if the sun is above you and you're looking down. Most often, they're seen from aircraft, but they can sometimes be seen in other situations.

14. ### gendankenRuler of All the LandsValued Senior Member

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Tavilich:
If you fly over the Pacific islands, the closer to the volcanic rim the more common you'd find those circular rainbows Dinosaur here's called "glories".

They'e said it already- its not a semicircle as much as it is the misofortune of location- location location location. With the added bonus of light refraction being linear.

Now- get you a hose, not the cheap kind you get in retail stores but the heavy duty kind you find in Home Deopts- go out at noon and set the nozzle to mist and over year head you'll find a circular rainbow cascading down on your head. Its Netwon's experiment raining down on you.

As for the sun, courtesy of some low budget website I just googled for:

"The disk is actually the same size throughout the day. It is an optical illusion. The Sun and the Moon both appear bigger when low on the horizon because you have foreground objects close by to allow you to compare sizes.
The colour of the Sun changes due to the atmosphere. If you take a look through a prism, or onto a compact disc, you can see a variety of colours that makes up white light. All these colours bend by different amounts, with blue light bending best. Red light hardly bends at all. Yellow light bends more than red but not as much as blue (blue light bends best!) - yellow is midway between the two. When the Sun is low on the horizon, most of the other colours are bent away from our line of sight. All apart from the red colour.
When we actually see the Sun low on the horizon, it has already gone below the horizon! But the red light is bent so much, that it is bent by the atmosphere around the Earth, and into your eyes. "

15. ### eburacum45Valued Senior Member

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In fact the Sun is slightly smaller at sunset than when it is high in the sky;
the image of the sun is the same width, but is somewhat flattened by refraction.

So there you go.
__________________
SF worldbuilding at
http://www.orionsarm.com/main.html

16. ### DinosaurRational SkepticValued Senior Member

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The following is a partial explanation of the cause and shape of a rainbow.

You see a rainbow when the sun is behind you and there is rain in front of you.

Rainbows and glories are created when light enters a spherical raindrop and is reflected twice internally. Light exits the raindrop directed approximately back toward the light source. Visualize the following.
• The sunlight originates behind your back, and can be thought of as parallel rays because the sun is so far away.
• Certain raindrops are in just the correct position relative to you and the sun. The correct locations happen to be on a circular arc (or a complete circle), which is why the rainbow is a circular arc and the glory is a circle.
• Each light ray (or photon) enters an approximately circular raindrop at a point on the hemisphere facing you, and exits from a point on the hemisphere facing you.
• After entering the raindrop, the ray (photon) is reflected twice internally on the interior of the hemisphere facing away from you.
• Since light travels slower in water than in air, there is refraction at the entry point and the exit point.
• Different colors have different wavelengths (energies), causing the separation of colors when passing from one medium to another.
Note that a wave theory talks about wavelengths, while a particle view talks about photons energies.

A search of the internet should result in some diagrams and better explanations.

17. ### John ConnellanValued Senior Member

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Believe it or not, this science is the basis for seeing color in pools of oil. The light is also reflected back twice from a film of oil (one from the top and one from the bottom) and one ray gets refracted causing the light to appear colourful the way Dinosaur says above.

18. ### narayannirRegistered Member

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Actually,rinbow is a result of refraction of light suspended water particles after a rainfall.These water droplets are present at a uniform height above the earth's surface.Since the earth is spherical in shape,these particles are present in a spherical plane,enveloping the earth..If such a plane of moisture is cut by sunlight,a circular line or the rainbow is generated..But to us,only one half of the rainbow is visible as our vision is restricted to the left and right horizon...Thatz why the rainbow appears semi-circular.

19. ### wlminexBannedBanned

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. . . now that we all understand rainbows . . . .and that we are only seeing the 'visible part' (colors) of the electro-magnetic (EM) spectrum . . . now visualize (out-of-the-box exercise here) that the 'entire rainbow' actually includes the non-visible parts of the EM spectrum as well . . . we just don't see these portions because they are not 'visible' . . . but they are there regardless! . . . makes for a much more EM-colorful rainbow. This visualization is helpful in considering a lot of things about the universe that 'appear' to be invisible (not normally detected!).

20. ### EmilValued Senior Member

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I have seen one similar.
I had the impression that if I jump high enough, I reach it.

21. ### whynotRegistered Senior Member

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I caught this partial rainbow not long ago.

22. ### Veselin AndreevRegistered Member

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this is an intriguing question are more to that the pic is awesome...!
but are there any serious answers..?:bugeye: